Delegated cronjobs with time vars per target host

[Martin Božič]

Hi,

I want to dynamically every new host to the backup server with rsnapshot. I
have no problems putting SSH keys on backup target hosts and putting a template
and exclude config for the target host into /etc/rsnapshot.d on backup host.

The problem is when I try to add the rsnapshot cronjob for the new host. And
here's the trick - I'm trying to add different times for each host because I
don't want them to all hit the backup disk at the same time. So I figured I
could put several cron time vars for each host in host_vars/ directory. Here's
an example of what I'm trying to pull-off with cron module:

FILE: roles/common/backup.yml
-----------------------------
- name: weekly rsnapshot cron
cron: name="rsnap {{ ansible_fqdn }} (upravlja Ansible)"
weekday={{ rsnap_weekly_day }} hour={{ rsnap_weekly_hour }}
minute={{ rsnap_weekly_min }} user="root"
job="rsnapshot -c /etc/rsnapshot.d/{{ ansible_fqdn }}.conf weekly"
cron_file="rsnap-{{ ansible_fqdn }}"
delegate_to: backuphost

FILE: host_vars/newhost
--------------------
rsnap_hourly_min: 12
rsnap_daily_min: 45
rsnap_daily_hour: 1
rsnap_weekly_min: 30
rsnap_weekly_hour: 21
rsnap_weekly_day: 3

What happens is that backuphost picks up the var of newhost and then cries
foul:

fatal: [backuphost] => One or more undefined variables: 'rsnap_daily_hour'
is undefined

I'm running out of ideas how I could do it. I've looked into several loops but
I'm still a beginner with Ansible and YAML syntax, so if anyone has any idea how
this delegation could work I'd really appreciate it!

Regards,
Martin

[Brent Langston]

I did this a while back, thinking I'd replace it with something more elegant when I have time... honestly, it's been working so well, I haven't thought about it again, until you reminded me.

I have a script saved in my ansible git repo: $GIT/ansible/bin/cron_scheduler.sh

#!/bin/bash

PATH=/bin:/usr/bin

helptext() {

          echo "Usage: $0 -i ip_address -f frequency" >&2

          echo "Example: $0 -i 10.10.5.160 -f 15 # run once every 15 minutes" >&2

            exit 1

}

if [ "$#" -ne 4 ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]; then

        helptext

fi

while getopts i:f: option

do

        case "${option}"

        in

                i) ip_address=${OPTARG};;

                f) frequency=${OPTARG};;

                *) helptext;;

        esac

done

decimal=$(echo $ip_address | sed 's/\./+/g' | bc)

ip_rand=$(expr $decimal % $frequency)

x=$ip_rand

declare -a minutes=$ip_rand

count=0

while [ $x -lt 60 ]

do

        next=$(expr "${minutes[$count]}" + $frequency)

        if [ $next -lt 60 ] ; then

                minutes=("${minutes[@]}" "$next")

        fi

        (( count++ ))

        x=$next

done

echo -n ${minutes[@]} | tr " " ,

So running this:

cron_scheduler.sh -i 10.10.5.160 -f 15

results in this output:

5,20,35,50 

(every 15 minutes) 

So just decide how frequently you want to schedule the task, and pass in the IP address.  voila. 

- name: pick a minute to run example

  script: bin/cron_scheduler.sh -i {{ ansible_default_ipv4.address }} -f 10

  register: cron_minutes

- name: configure example

  template: src=example/example.cron dest=/etc/cron.d/example.cron owner=root group=root mode=0644

and the template would have something like this:

{{cron_minutes.stdout}} * * * * root /usr/local/bin/example

--------
Brent
--------

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[Brian Green]

Where are you using the rsnap_daily_hour variable? The task you pasted doesn't have it included.  

The vars file you included is host specific for 'newhost', if you are trying to use those variables in a play involving the backuphost (not a delegated_to task), you'll either need to use hostvars or restructure your variables in a different manner.

So, using hostvars, accessing rsnap_daily_hour in a play on backuphost would be {{ hostvars['newhost']['rsnap_daily_hour'] }}.

If it was me, I'd use group_vars with separate groups for each time slot, and then make the backuphost a member of each time slot group.  That may or may not scale well, but I try to avoid using hostvars if at all possible. I'd change my mind if I was in an environment absolutely destined to remain small & stable of course.

Brian

[Michael DeHaan]

There have been pull requests for a random Jinja2 filter, but they were pretty complicated in terms of syntax (i.e. not pretty syntax wise) and I wanted to see them pared down.

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